Curt - we did a LOT of research on the 2-barrel to 1-barrel adapters several years ago. We found the following to be true.
(A) under no circumstances would any two-barrel be an improvement over a single 1-barrel that was the proper size for the hole in the manifold.
(B) to minimize losses of using a two-barrel to one-barrel adapter, the adapter needed to be shaped for the transission, and at least 4 inches tall.
If possible, cut the one barrel mounting from the plenum area of the repop manifold, and heliarc a two-barrel flange that matches the carburetors of your choice into the plenum. Make certain that the flange is centered into the plenum, and that the plenum is sufficiently large to accept the two barrel flange.
If you simply want the two barrel(s) for looks and don't mind the loss of performance, go for it.
Probably need to qualify for what we were testing. We tested for an improvement in horsepower and/or driveability. We did not test for fuel economy.
Jon.
